Process of producing cellulose from lignified material



May 12, 1931. R. B. WOLF ET AL PROCESS OF PRODUCING CELLULOSE FROM LIGNIFIED MATERIAL Filed June 13, 1950 INVENTOR new Y ATTORNEY K2621 mw mphw w BY mummbmum twzmsm Patented May 12, 1931 .UNITED STATES PATENT OFFICE ROBERT B. WOLF, OF NEW YORK, N. Y., ANn RAYMOND R HILL, or GLEN Roox, AND

RAYMOND s. HATCH, or EAsT RANGE, NEw JERSEY, AssroNoRs TO LIGNO-CELLU- LOSE cORroRATION, on NEW YORK, N. Y., A CORPORATION OF DELAWARE PROCESS PRODUCING-.CELLULOSE FROM LIGNIFIED MATERIAL Application ,flled June 13, 1930. Serial li o. 460,976.

- ed United States Patent No. 70,483 for the production of cellulose from wood and other ligneous material, by heating the wood under pressure with a solution of sulphur dioxide. Early in his experiments with solutions of sulphur dioxide, Tilghman found that the produced was brown in color and not leachable. vThis brown color was, he 'assumed, caused by the presence offree sulphuric acid and this led him to add a certain amount of such bases as calcium or magnesium hydroxide to the sulphurous acid so-' lutions, to overcome the harmful effect of the sulphuric acid present. As a result of Tilghmans experience, definite amounts of bases, such as calcium or magnesium in combination with more or less sul hurous acid have been used in the practice 0 the so-called sulphite process.

The object of this invention is a provide an improved and simplified procedure for the recovery of cellulose from available cellulose-containing materials.

A further object'of the invention is to efl'ect a material saving in sulphur and steam,

as compared with the sulphur and steam consumed in producing cellulose by the well known bisulphite process.

Another object of the invention is to provide for the convenient recovery of the excess of sulphur dioxide used in the process andto render the recovered gas immediately available for further operation of the process.

Another object of-the invention is to produce a sulphonated lignin compound, tree from inorganic bases and polymerized to a minimum extent, which will serve as a raw material for the production of valuable organic by-products.

In studying the efiectofstrong solutions of free sulphur dioxide, we have discovered that if concentrations exceeding 12% free S0 are used we obtain extremely rapid penetration of the wood substance, and that cook ing can be carried on at temperatures much lower than those used at present, rarely ex ceeding 100 C. We have further discov ered that with concentrations of S0 in excess of 12%, no base is necessary and with the proper recovery methods the sulphur consumption is much less than in the case of the sulphite process as now carried on.

We have discovered, however, that when working with solutions of sulphur dioxide of 12% concentration, or greater, the technology of-the acid "preparation and gas recovery demands methods and equipment quite different from those in use in the well 'known bi-sulphite process, and the various steps in the cooking of the raw material and recovery of the gas must be carried out in a manner entirely unique.

We have also discovered that at pressures of approximately two or three atmospheres both sulphur dioxide gas and liquid are miscible with water, in all proportlons, at temperatures normally encountered in the recovery process. In order, therefore, to easily and economicallyrecoverthe bulk of the, excess gas left in the digester after the cooking process, it is only necessary to relieve this gas into another vessel, containing water or a weak solution of S0 at a pressure of 40 to pounds per square inch. The gas remaming in the digester when the pressure has been lowered to 4050 pounds may then be removed, cooled, and absorbed] in water or weak S0 solution, which will later serve as the solution in which subsequent large amounts of SO may be absorbed at t0 to 50 pounds pressure.

It is important to avoid elevated temperatures in the process of removmg the residual sulphur dioxide from the contents of the digester after the been recovered. or this reason we prefer to relieve down to atmospheric pressure and, subsequently, to reduce the pressure in the digester below atmosphere, by means of a ain portion of the gas has 7 vacuum pump, which will accomplish commixture of cellulose and sulphonated lignin remaining in the digester.

After removing all possible free sulphur dioxide, there remains in the digester a solution of sulphonated lignin and the cellulose present in the lignified raw material. This mixture is flushed out of the digester and the sulphonated lignin solution separated from the cellulose. The sulphonated lignin solution is available for the production of valuable by-products.

In order to provide strong sulphur dioxide solutions that will be available for charging digesters after a shut down of the plant, or to supply a compensating amount of strong sulphur dioxide to overcome moisture variations in the ligno-cellulose raw material, we prefer to prepare continuously a definite excess of sulphur dioxide over that necessary for actual combination with the lignin. During the relieving of the sulphur dioxide, after the eookingis completed, a definite amount of this excess gas is relieved into a storage vessel containing water and maintained at a pressure of from 4:5 to 55 pounds per square inch. Under these conditions the solution may be built up in sulphur dioxide content to any desired concentration, since at 55 pounds per square inch pressure and normal temperatures pure sulphur dioxide and water are miscible in all proportions.

Tn the accompanying drawing we have shown diagrammatically an apparatus adapted for carrying on the digesting of the raw material with strong solutions of tree SU and for the recovery of the unconsumed S Referring to the drawing, a sulphur burner 1 with its combustion chamber is connected to a gas cooler 2. The gas is delivered to a scrubber 3 in which a soluble bi-sulphite is circulated to remove any SQ- present by a double decomposition, liberating S0 to replace the SO A blower 4 capable of raising the gas pressure to 20 to 25 pounds per square inch forces the gas into an absorbing tower 5, which isfilled with inert material. lVater flows down over this inert material and presents a large absorbing surface to theupward flowing gas. The pressure in the tower 5 is limited by a pressure regulating valve 6. Weak acid flows through a valve 7 into one of the series of storage tanks 8. The w cak acid is pumped by means of pump 9 through -valve 31 into a digester 10 containing the charge of raw material, wood chips, for example. The weak acid 1s brought up to proper strength by means of very strong acid from storage, or other suitable source, after which steam is admitted to the coils 11. The

pressure in the digester 10 rises and air and weak gas are allowed to escape. through the Valve 13, to the tower 5. lVhen the air has all been relieved from the digester 10, the valve,

recess? pressure, which pressure must exceed six atmospheres and is preferably in excess of fifteen atmospheres. The contents are kept at that point until the lignin has combined with all of the S9 it will take up. The steam is then shut off and valves 14 and 15 are opened, admitting S0 to the digester 16, which contains a charge of raw material and weak acid from the storage tank 8. At the same time the valve 17 is opened to allow a definite percentage of the gas from the digester 10 to pass to the storage tank 18. When the pressure in the digester 10 drops to about 55 pounds, the valve 17 is closed and relief into the digester 16 is continued until the pressure in the digester 10 is down to 35 to 10 pounds. The valves 14: and 15 are then closed, the valve 19 is opened, and the gas passes through the cooler 20 and pump 21 into the weak acid in the tank 8, until a slight vacuum is produced in the digester 10. Alternatively, it a pump capable of overcoming sufficient pressure is employed, the gas may be pumped through valve 17 into storage vessel 18. The valve 19 is then closed and the contents of the digester 10 are discharged through the valve 29. Steam is then admitted into the coil 27 in the digester 16. Air is relieved through the valve after which the valve 25 is closed; valves 22 and 23 are opened and St): is admitted into digester 10, which contains a fresh charge of raw material and weak acid from the tank 8. Valves 12 and 24: are safety valves set at any predetermined pressures. Valve 26 takes excess S0 to the storage tank 18. Valve 32 conducts gas, after the pressure is reduced to to pounds in the digester 16, back to the Weak acid tank 8. The pressures are indicated by pressure gauges 33 and 34:.

As will be understood readily from the foregoing description, the procedure permits the preparation and maintenance of very strong solutions of sulphur dioxide containing upward from 12% of free S0 and the use of such solutions in the digesters. It provides nevertheless for the recovery of the sulphur dioxide which is relieved at the lower pressures. The use of strong solutions of sulphur dioxide in the digester ensures the most effective cooking at relatively low tem peratures, thus avoiding polymerization of the sulphonated lignin, and saving'a considerable quantity of steam, indirect heating being employed preferably. llt likewise avoids the necessity for the presence of bases in the cooking liquor which in turn permits the recovery of lignin constituents free from inorganic bases.

Various changes may be made in the details of procedure and apparatus employed without departing from the invention or sacrificing any of the'advantages thereof.

We claim: I I

1. The method of recovering cellulose from cellulose-containing material which comprises digesting the material at a pressure exceeding six atmospheres with a water solution free from inorganic bases and containing more than 12% of free S0 2. The method of recovering cellulose from cellulose-containing material which comprises digesting the material at a temperature below 100 C. and a pressure exceeding six atmospheres with a water solution free from inorganic bases and containing more than 12% of free S0 3. The method of recovering cellulose from cellulose-containing material which comprises digesting the material with a water solution free from inorganic bases and con taining more than 12% of free S0 and absorbing the relief gases at a pressure above 40 pounds per square inch in water under such pressure.

4. The method of recovering cellulose from cellulose-containing material which comprises di' 'esting the material with a water solution ree from inorganic bases and containing more than 12% of free S0 absorbing the relief gases at a pressure above 40 pounds per square inch in water under such pressure, and absorbing the balance of the relief gases at atmospheric pressure.

5. The method of recovering cellulose from cellulose-containing material which comprises digesting the material with a water solution free from inorganic bases and containing more than 12% of free S0 absorbing the relief gases at a pressure above 40 pounds per square inch in water under such pressure, absorbing the balance of the relief gases at atmospheric pressure, and finally subjecting the cooked material to sub-atmospheric pressure to remove any remaining free sulphur dioxide. 1

6. The method of recovering cellulose from cellulose-containing material which comprises digesting the material at a pressure exceeding six atmospheres with indirect heating in a water solution free from inorganic lsuges and containing more than 12% of free 7. The method of recovering cellulose from cellulose-containing material which comprises digesting the material at a temperature below 100 C. and a pressure exceeding six atmospheres with indirect heating in a water solution free from inorganic bas s and containing more than 12% of free S02.

8. The method of recovering cellulose from cellulose-containing material which comheating in a water solution free from inorganic ases and containing more than 12% of free S0 absorbing the relief gases at a pressure above40 pounds per square inch in water under such pressure, and absorbing the balance of the relief gases at atmospheric pressure.

10. The method of recovering cellulose from cellulose-containing material which comprises digesting the material with indirect I heating in a water solution free from inorganic bases and containing more than 12% of free S0 absorbing the relief gases at a pressure above 40 pounds per square inch in water under such ressure, absorbing the balance of the relic? gases at atmospheric pressure and finally subjecting the cooked material to sub-atmospheric move any remaining tree sulp ur dioxide.

In testimony whereof we afix our signatures.

ROBERT E. WULF. RAYMOND P. HILL. RAYMOND S. HATCH.

prises digesting the material with indirect heatin in a water solution free from inorganic ases and containing more than 12% of free S0 and absorbing the relief gases at a pressure above 40 pounds per square inch in water under such pressure.

9. The method of recovering cellulose from cellulose-containing material which comprises digesting the material with indirect pressure to re 

